About magnetic tapes that have been used for data storage for computers in recent years, the recording density thereof has been improved to increase the recording capacity. For this purpose, in a magnetic tape having a data track in the longitudinal direction, the track width of the data track, in which data are to be recorded, becomes very narrow. For example, a recording format that the track width is 20 μm or less is suggested. When the data track width becomes narrow in such a way, it is very difficult to match the position of the data track on the magnetic tape and that of a signal reading element of a magnetic head with each other; thus, the following manner is adopted: a servo signal, which will be a positional benchmark, is written beforehand on a magnetic tape; swing of the tape in the width direction is detected by reading out the servo signal when the magnetic tape runs on a magnetic tape drive; in accordance with the swing, the magnetic head unit of the drive is moved in the width direction of the tape by means of an actuator, thereby attaining feedback control; thus, the signal reading element of the magnetic head is caused not to get out of the data track even if the width of the track is narrow.
Since the servo signal written in the magnetic tape is at a position for a benchmark for recording of data signals onto the magnetic tape and reproducing of them, it is necessary to record the servo signal precisely onto the magnetic tape in the producing process thereof. For this purpose, in the producing process of the magnetic tape, a fixed guide having a regulating unit for regulating the magnetic tape in the width direction is arranged adjacent to the magnetic head. The magnetic head is fixed in this way so as to be hardly swung in the width direction. This magnetic head is used to write the serve signal. A servo writer exclusive for the writing is used. The magnetic tape on which the servo signal is written in this way is stored into a cartridge case to produce a magnetic tape cartridge for computers.
However, generally-used magnetic tape drives do not have any running system the precision of which is equivalent to that of servo writers from the viewpoint of costs and the durability of magnetic tapes against repeated use thereof. Even if a servo signal is precisely written on a magnetic tape, the magnetic tape is swung in the width direction thereof when the tape runs in a drive, so that servo control cannot follow the movement of the tape. As a result, errors of reading out data may be generated.
A specific example of the servo writer is described in Patent Document 1 (JP-A-2005-92938), and a specific example of the magnetic tape drive is described in Patent Document 2 (Japanese Patent No. 2729031).